1. Technical Field
The present invention relates to an electric device and a diagnostic apparatus. More particularly, the present invention relates to an electric device that operates in synchronization with a clock signal and to a diagnostic apparatus that diagnoses such an electric device.
2. Related Art
A known diagnostic apparatus operates an electric device including a plurality of circuits, acquires at a given timing an output value from a target circuit selected from the plurality of circuits, and judges whether the target circuit is acceptable based on the acquired output value (for example, see Patent Document 1). Such a diagnostic apparatus stops the clock signal in the electric device at a timing at which a to-be-diagnosed circuit is desired to be diagnosed and reads a steady-state output value from the to-be diagnosed circuit while the clock signal remains suspended. The diagnostic apparatus allows the clock signal to be resumed after having read the output value.
An exemplary electric device is an LSI that operates with the GHz clock frequency. The LSI may include a timing adjusting circuit that adjusts a timing of exchanging data with a different LSI so as to satisfy an optimal margin. Such an electric device is diagnosed by a diagnostic apparatus that attempts to detect an optimal data exchange timing while sequentially changing a timing.
Patent Document 1: Japanese Patent Application Publication No. 10-096758
When a clock signal is stopped and thereafter resumed in an electric device, there is a large change in the power consumed by a clock buffer, a flip-flop, and other components of the electric device. Thus, turning on and off the clock signal in the electric device results in a large change in the power supply current, which significantly increases and decreases the power supply voltage.
As described above, when an electric device is subjected to a diagnosis process performed by a diagnostic apparatus of the type that makes a diagnosis based on an output value that is read from a to-be-diagnosed circuit of the electric device while a clock signal remains suspended from being supplied to the electric device, the power supply voltage of the electric device dramatically increases and decreases within a certain period of time, for example, several milliseconds. This dramatic change creates sudden increase and decrease in the temperature of the electric device. Thus, during the diagnosis process performed by the diagnostic apparatus of this type, the electric device operates with transient power supply voltage and temperature, which are different from normal power supply voltage and temperature observed during an actual operation. For this reason, the diagnostic apparatus cannot make a diagnosis based on stable data. The diagnostic apparatus may also read the output value from the to-be-diagnosed circuit while the clock signal remains suspended from being supplied to the electric device for another purpose of detecting a time margin for an input/output buffer or the like that enables the electric device to reliably exchange data with a different device. During such a time-margin detection process, the diagnostic apparatus also inevitably causes the electric device to operate under different conditions from when the electric device actually operates. Therefore, the diagnostic apparatus has difficulties in detecting an accurate time margin.
Furthermore, the electric device experiences a large change in the power supply load when the clock signal is suspended and thereafter resumed. Therefore, when the diagnostic apparatus successively performs a plurality of diagnosis processes on the electric devices, a period of time of approximately several milliseconds needs to be provided every time the clock signal is resumed to restore the power supply load back to a normal level.